Room dimensions for small listening rooms

Over the years, people have suggested certain ratios
of room dimensions that minimise the distortion caused by low frequency
room modes. These "golden ratios", however, are not necessarily
the best dimensions to choose for a room and below some new best
room dimensions are suggested.

These web pages gives some
more detailed numerical results from an Audio
Engineering Society journal paper which appeared in June 2004.
The journal paper gives more details of the method used and can
be downloaded from the AES web site.

Why choose the right room size?

The sound that is heard in a critical listening
room is determined by the combined effect of the electronics of
the audio system and the physical acoustics of the listening environment.
The tonal balance and timbre of a sound can vary significantly depending
on the placement of the listener and loudspeaker and the geometry
of the room. Indeed, the modal artefacts introduced by the room
can be so influential that they dominate the sound. We will concentrate
on the design challenges to minimize these artefacts at low frequencies.
Consequently, we are concerned with the interaction between the
sources, listeners and the room modes.

Modes in small rooms often lead to extended sound
decays and uneven frequency responses – often referred to
as coloration. Problems arise at low frequencies because of the
relatively low modal density. Designers try to overcome the problems
of modes by choosing an appropriately proportioned room, by placing
the listeners and loudspeakers in suitable positions and by using
bass absorbers. Here we will concern ourselves with the first problem,
choosing the right room dimensions.

The determination of appropriate source and receiver
location is often undertaken by a trial and error process. Although
this is a laborious task, it is possible to do. When sources or
receivers are moved, the frequency response changes due to the variation
in the modal pressure distribution in the room and the changing
radiation resistance of the source. By choosing correct positions
in the room, it is possible to minimise the audible effects of the
modes within a room . As well as considering the modal (steady state)
response, others have considered the effects of first order reflections
from boundaries. In particular, the first order reflections from
the nearest wall boundaries to the source have been considered along
with the effect these reflections have on the frequency response
(e.g. [1]). A more complete solution using optimization is possible
[2].

At low frequencies rooms suffer from
modal problems. The figure to the right shows a typical frequency
response. Whatever the shape of a room, there will be a low frequency
range where modes are rather sparsely spaced in frequency, leading
to a poor sound quality. In particular, it is the excessively long
sound decays of modes that make them stand out and more audible.
While this problem can be solved by damping the modes (by adding
absorption), it is best if the room has dimensions that help minimise
the modal problems in the first place, because this makes it easier
(and cheaper) to treat the modes.